Conventional fibrous reinforcement materials can improve the strength, rigidity, etc., of a resin by their presence. Furthermore, glass fibers, for example, have been widely used for such fibrous reinforcement materials. Such glass fibers, however, have disadvantages in that, due to the high specific gravity of the glass fibers, they do not satisfy a demand for the reduction in weight, and furthermore, since the ash content of the glass fibers when incinerated is substantial, the burden on the environment is heavy.
On the other hand, a fibrous reinforcement material using a cellulose fiber as a carbon-neutral biomass raw material has been proposed. Since cellulose is a principal component of cell walls constituting a plant body, a fibrous reinforcement material formed using such cellulose can contribute to resource savings since such a fibrous reinforcement material can be produced with, for example, a waste material of wood as a raw material.
For such a fibrous reinforcement material using a cellulose fiber, a composite fiber containing a cellulose fiber and a single-walled carbon nanotube has been proposed in Japanese Patent Application Laid-Open Publication No. 2011-208327. According to this publication, the mechanical properties, such as tensile strength, of the composite fiber disclosed therein can be substantially improved due to the containment of single-walled carbon nanotubes.
Carbon nanotubes, however, are hydrophobic and the affinity with liquids is low. In other words, the carbon nanotubes have the disadvantage of low dispersibility. Therefore, the composite fiber disclosed in this publication is disadvantageous because it is difficult to uniformly disperse the single-walled carbon nanotubes in the cellulose fiber and it is also difficult to improve the tensile strength, etc., thereof easily and reliably.
Furthermore, generally, carbon nanotubes do not possess straightness. Therefore, according to the composite fiber, precise orientation of the single-walled carbon nanotubes in an axial direction of the cellulose fiber is difficult. In this regard, the composite fiber is disadvantageous in that the tensile strength, etc., of the composite fiber cannot be improved easily and reliably.
There remains a need for cellulose fiber that can reduce environmental burden while still enhancing modulus of elasticity easily and reliably. The present development addresses this need.